Estimation of proportions of different pulp types in recycled paper and deinked pulp by acid methanolysis

We have developed and evaluated a method to estimate the proportion of bleached kraft pulp from hardwoods (HP), bleached kraft pulp from softwoods (SP) and mechanical pulp (MP) in deinked pulp (DIP) or recycled paper (RP) by analysis of the sugar units in hemicelluloses and pectins. Different pulps, i.e., HP, SP and MP, were obtained from Nordic pulp mills. The pulps were mixed in different proportions and dried. The content of arabinose, xylose, mannose, galactose, rhamnose, galacturonic acid and 4-O-methyl-glucuronic acid in hemicelluloses and pectins in the different pulps and in the mixtures were determined by acid methanolysis and gas chromatography (GC). The contents of the sugar units were significantly different in the three pulp types, but quite similar in the same pulp type, irrespectively of bleaching sequence. A linear model for estimating the relative proportions of HP, SP and MP in DIP or RP were formulated. Hence, given a predefined set of errors, ε_i, and data of HP, SP and MP, the relative proportions can be estimated by solving a linear least-squares problem. It was concluded that the proportion of hardwood kraft pulp, softwood kraft pulp and mechanical pulp in deinked pulp or recycled paper can be estimated using a mathematical model based on the contents of the sugar units in hemicelluloses and pectins. The difference between the actual mixing ratio and the estimated values was on average less than ± 5%.